The safety of an aircraft during take-off is critically dependent upon the integrity and smoothness of the aerodynamic and control surfaces which generate the lift necessary to render an aircraft airborne at sufficient ground speeds. During adverse weather conditions--in which snow, ice and/or frost can accumulate on the aerodynamic or control surfaces of an (aircraft--it is necessary to restore and maintain the aerodynamic integrity of the aircraft to assure the safety of the passengers and crew.
The regulations concerning the restoration of aerodynamic integrity under icing and other adverse weather conditions (relating to aircraft performance and flight characteristics) were established as early as 1950 by the Civil Aeronautics Board and remain in effect today under Federal Aviation Regulations (FAR) sections 91.209, 121.629, and 135.227.
In addition, recent FAA Advisory Circulars also recognize the importance of the removal of underwing frost (as, for example, FAA Advisory Circular 20-117, Dec. 17, 1982, appendix 3, paragraph 3.b.(3)). Paragraph 3b(1) of this circular further recognizes that de-icing (i.e., the periodic removal of ice) and anti-icing (i.e., preventing ice formation) are distinct and separate components in the certification of aircraft for flight in icing and other adverse weather conditions. The Federal Aviation Administration, therefore, recognizes the importance of addressing each of these issues when considering flight safety in icing and other adverse weather conditions.
Moreover, it is necessary to carefully consider the various types of ground icing which can affect an aircraft in different ways by the formation of stronger or weaker bonding of the ice layer to the surfaces of the aircraft. Each of these considerations can require different application techniques in order to assure the safe, complete, efficient and economical removal of each type of ice.
There are three major types or groups of ground icing deposits or formations.
The first group ("Group I") is produced by the sublimation of water into ice while omitting the vapor phase. Group I includes hard (crystalline) ice, glaze ice and rime ice. Hard (crystalline) ice is formed when oncoming masses of warm air interact with the surface of objects that are already at a lower sub-zero temperature. Glaze ice is formed in severe frost as a result of the oversaturation of air with water vapor. Rime ice is formed in calm, clear weather by the emission of heat from a surface which is at a lower sub-zero temperature than the surrounding air. Air near these surfaces is cooled to a point in which the inherent water vapor quickly reaches a saturation level and is converted directly into ice at the coldest point which is the surface-to-air interface. These three types of Group I deposits are all snow-like in appearance, unstable, and of low density; thus may be easily removed from the airfoil and other aircraft surfaces.
The second group of ground icing types ("Group II") results from the presence of supercooled water in the atmosphere. Under these conditions, ice is formed as a result of the crystallization on the aerodynamic and control surfaces of supercooled droplets of rain, mist or drizzle. Air temperatures of approximately 0.degree. C. are usually present with this type of ice formation on the ground. The ice deposits of Group II bond much more firmly to the aerodynamic and control surfaces of the aircraft (than the sublimated deposits of Group I) and may attain very large dimensions.
The third group of ground icing types ("Group III") includes ice formation produced by "ordinary non-supercooled" water formed on the aerodynamic and control surfaces of the aircraft. Ordinary non-supercooled water may include (but is not limited to) rain, wet snow, precipitated fog droplets, and water vapor condensate. Their outward appearance may be similar to Group I and Group II type ice deposits; however, Group III type ice deposits form a solid bond with the aerodynamic and control surfaces of the aircraft (as referred to by Dr. O. K. Trunov, Aviation Week, June 1985, pages 17-21).
It would be very desirable and important, therefore, to have an apparatus and a method which can modify fluid and fluid/air application procedures necessary to de-ice and anti-ice aircraft under the various conditions which lead to the three different ice group formations. This would assure the restoration of proper flight characteristics in the most efficient and cost effective manner possible.
The existing regulations which govern these proper flight characteristics (and which are specified hereinbefore) are based on the principles and regulations which are collectively known in the airline industry as the "Clean Aircraft Concept". This concept is based on Federal Aviation Regulation 91.209(a)(2), which states that:
"No pilot may take off an airplane that has . . . snow or ice adhering to the wings, or stabilizing, or control surfaces . . . ". PA1 K=a constant PA1 L=lift PA1 A.sub.W =wing area PA1 S=speed. PA1 "Wind tunnel and flight tests indicate that ice, frost, or snow formations on the leading edge and upper surface of a wing, having a thickness and surface roughness similar to medium or coarse sandpaper, can reduce wing lift by as much as 30 percent and increase drag by 40 percent.".
The principles known as the Clean Aircraft Concept acknowledge the known degradation in aircraft performance and changes of aircraft flight characteristics when ice formations of any type or group are present. Under normal conditions, the airflow over a wing smoothly follows the shape of the airfoil, thereby providing the "lift". The lift is defined as the force generated by the flow of air over a lifting surface. The lift varies directly with the "angle of attack", which is defined as the inclination of the fuselage reference plane ("FRP") to the oncoming airflow. As the angle of attack increases, it becomes more and more difficult for the air to follow the airfoil shape; and the air begins separating from the wing. When the flow of air is fully separated from the wing, the wing is considered to be "stalled"; that is, the lifting capability of the wing is fully degraded. The "lift coefficient", which is defined as a non-dimensional parameter that allows the comparison of the lifting ability independent of size or speed, declines sharply.
The lift coefficient can be described by the equation: ##EQU1## where: C.sub.L =lift coefficient
This equation shows that anything which affects the wing area will have an inversely-proportionate impact on the lift coefficient. The formation of ice or other materials which increase surface roughness impacts upon the effective wing area. Again, in turn, the normal variation of lift with the angle of attack can be significantly and adversely altered by the presence of ice or other contamination. Specifically, the effect of ice or other material contamination of wing surfaces is to reduce the maximum lift capability of the wing and to cause stall to occur at a lower angle of attack. In other words, a contaminated wing could cause an aircraft to stall at an otherwise normal departure angle from the runway.
Ice or other materials which contaminate the wing surface also increases the "drag" or "drag coefficient" (herein defined as the retarding force exerted on a moving body by a fluid medium). It does so by increasing surface roughness and disturbing the smooth airflow necessary for lift over the airfoil surface. Even small accumulations can have a dramatic impact on flight characteristics. As stated in the FAA Advisory Circular 20-117 (dated Dec. 17, 1982):
It will be appreciated that the degradation of aircraft performance and flight characteristics due to an increase in the drag coefficient have an impact similar to the reduction of the lift coefficient (as discussed hereinbefore) and thus will not be repeated herein (reference being made to Aircraft Ground De-icing Conference Proceedings, Sept. 20-22, 1988, R. E. Brumby, pp. 47-66).
Even under ideal weather conditions, the accumulation of dirt, sand, insects, airborne particles, dirt entrained oil droplets, and surface irregularities which result from high speed impact of minute particles can all adversely affect the lift coefficient on an airfoil surface. Thus, it is also necessary to regularly wash the aircraft to maintain optimal flight characteristics (as well as enhancing the appearance of the aircraft).
Further, the misapplication of de-icing and/or anti-icing fluids--especially recently developed fluids of higher viscosities (including those still in research and development)--may cause ripple effects on the wing surface during take-off; and as a result, drag forces are increased, and the lift coefficient of the airfoil is reduced. Prior and current cleaning, de-icing and anti-icing technologies (to be described hereinafter) delivering these new generation fluids (also to be described hereinafter) to maintain the manufacturer's design specifications on viscosity and coverage are, nevertheless, inefficient and uneconomical.
Early aircraft precluded operations in icing or weather conditions due to a lack of airborne navigational reliability; therefore, icing concerns were a moot point. As avionics improved, however, weather conditions became more of an operational factor.
The use of hangers, wing and/or component covers to avoid exposure to the elements lessened but did not eliminate the work needed to remove ice, snow or frost. At first, the use of simple tools for the direct removal of accumulations of ice, wet snow and frost from aircraft surfaces included brooms, brushes, ropes, squeegees, fire hoses and other devices. Albeit functional, these initial methods were crude, time consuming and inefficient; and as the size of the aircraft increased, these manual methods became totally unsatisfactory.
Moreover, significant drawbacks are inherent in the use of these simple tools and devices. First, care has to be exercised when using these tools so that the aircraft skin and other critical components are not inadvertently damaged during removal operations. Second, these methods are generally useful only during clear, cold weather with dry snow or frost accumulations. As the severity of the weather increases, the continual precipitation (in the form of wet snow, freezing rain or sleet, etc.) becomes a definite factor in the preparation of the aircraft; and thus the mere removal is not sufficient to maintain the Clean Aircraft Concept necessary to approve the aircraft for take-off. In addition, the introduction of larger aircraft, as well as the rapid increase in scheduled flights, further aggravated the existing problem.
Basically, the problem is reliable scheduling of aircraft, fast "turn around", and efficient use of facilities and manpower, consonant with the highest standards of aircraft safety under inclement weather conditions.
In an effort to alleviate this problem, freezing point depressant ("FPD") fluids consisting substantially of organic alcohols were introduced quite some time ago (approximately during the late 1940's, early 50's) primarily for de-icing purposes. FPD fluids take advantage of the eutectic point phenomenon of certain organic solvents. In this phenomenon, two completely miscible solvents can be combined in varying proportions at constant pressure to lower the freezing point of the mixture below the freezing point of either component, since each solvent interferes with the crystallization of the other solvent at their respective normal freezing points. The components of all commercially available FPD fluids are of the ethylene glycol and propylene glycol family. The ethylene glycol fluids are designated "Type I", and the propylene glycol fluids are designated "Type II". Exact formulations (including corrosion inhibitors, wetting agents and more recently viscosity enhancing agents) are proprietary to the manufacturer. However, all of these FPD fluids exhibit this eutectic point phenomenon when mixed with water. It is generally accepted from experimental and actual operating data that the minimum freezing point occurs when the mixture is approximately sixty percent (60 %) glycol and forty percent (40%) water. The addition of either water or glycol at this point will raise the minimum freezing point.
The application of aircraft de-icing fluid ("ADF") and/or FPD fluids to the aircraft surface can utilize anything from very simple manual techniques (mops, buckets, brushes, and hand pumps) to more elaborate mobile platform-mounted spraying equipment operated by ground support personnel.
The most common equipment presently utilized for aircraft de-icing is based on technology that is approximately thirty (30) years old. Basically, the de-icing equipment is mounted on a truck chassis and requires two operators. Examples of this type of two-man mobile de-icing equipment are the model D-40-D truck manufactured by the Ted Trump Company (and capable of delivering up to 1800 gallons of FPD mixture) and the truck models TM-1800 and LA-100 manufactured by the Airline Equipment Division of the FMC Corporation of Orlando, Fla. (and capable of delivering 1800 and 1000 gallons of FPD mixture), respectively.
The procedure is as follows: Once the truck is properly positioned in relation to the aircraft, a boom device operates from the top of the truck, thereby spraying a hot liquid onto the aircraft to de-ice (or clean). The truck is repositioned around the aircraft as each section is de-iced (or cleaned) until the entire aircraft has been completed. When necessary, the truck makes a second circuit of the aircraft to apply another coating of the FPD fluid for anti-icing protection.
While functional, this existing procedure is inefficient in terms of the investment in specialized equipment, manpower requirements, time, glycol consumed, and the effects on the environment. To de-ice and anti-ice a narrow-body aircraft (e.g., a Boeing 727 or DC-9) a typical two-man team under adverse weather conditions takes approximately 10 to 12 minutes, and the operation can consume in excess of 1000 gallons of glycol at a cost of over $3,000.00. The equipment (which is of limited use) represents an investment of $200,000 or more per unit and is restricted to de-icing and anti-icing aircraft with occasional maintenance usage of the boom assembly for access to the upper aircraft components.
The additional processing time per aircraft necessary for this type of operation during adverse conditions extends the scheduled flight time for passengers. In addition, de-icing personnel are frequently drawn from other essential functions. This can cause further delays as original tasks become understaffed. As delays accumulate, the flight crew can exceed their allowable safe flight time as established by the FAA. Once allowable flight time is exceeded, the original flight crew must be replaced by another flight crew which only further adds to the delays and expense of operations. As the first flight for an aircraft is delayed, its remaining schedule must be adjusted. Thus, each delay cascades through an airline's schedule and accumulates added costs and customer dissatisfaction until the adverse weather conditions subside.
Finally, most de-icing is performed at the gate. At this location, the amount of ethylene glycol used is excessive and wasteful. The excess fluid at this location is allowed to flow into the storm drain systems of the airport. Thus, this material is not only lost for recycling purposes, but it also adds a toxic pollutant to the environment.
In an effort to solve these problems and disadvantages of long standing, the patented prior art has suggested various devices and arrangements which utilize towers that include booms and systems for glycol recovery and re-use. These devices and arrangements, of which we are aware, are represented by the following U.S. Letters Patents:
______________________________________ Inventor(s) U.S. Pat. No. ______________________________________ Yaste 3,533,395 Cook 3,612,075 Arato 3,835,498 Magnusson et al 4,378,755 Magnusson 4,634,084. ______________________________________
For example, U.S. Pat. No. 4,378,755 to Magnusson et al discloses a pair of spaced-apart portals through which the aircraft passes. Each portal supports a multiple of specially-designed conduits which is provided with a plurality of nozzles in the plane of the conduit. One of the conduits (intended to service the largest aircraft) is fixed. All of the other conduits are intended for respective aircraft (to conform to the profile of the respective aircraft) and are raised and lowered as required. Thus the system is inflexible and/or inefficient for other present aircraft or for future aircraft. An underground conduit has upwardly-directed nozzles to service the under surfaces of the aircraft. All of the nozzles are controlled by sensors. Since the portals are spaced apart, the first portal sprays hot water onto the aircraft for washing off the snow and ice. After the snow and ice is thus removed from the aircraft, the aircraft goes through a second portal; and this second portal showers a concentrated glycol onto the aircraft to prevent a new coating of snow and ice until the aircraft is airborne.
While these prior art devices and arrangements are intended to improve efficiency in terms of manpower and glycol consumption, each nonetheless still includes various drawbacks that have frustrated their widespread acceptance and commercialization.
In particular, none of the prior art devices or arrangements (of which we are aware) permits the aircraft to be both de-iced and anti-iced, conveniently and efficiently, and in one "pass" through a single integrated apparatus, thereby conserving manpower and glycol consumption. None of the prior art devices and arrangements is bidirectional, meaning that the aircraft may not enter into the apparatus from one of two opposite directions. None of the prior art devices or arrangements is capable of quickly and easily modifying the fluid(s) and fluid(s)/air application procedures thereof, thereby efficiently de-icing and anti-icing an aircraft under the different ice group formations experienced under various adverse weather conditions. None of the prior art devices or arrangements is capable of consistently and efficiently applying the de-icing and/or anti-icing fluid(s) of higher viscosities in such a manner so as to maintain the design specifications on viscosity and coverage that are necessary to prevent the ripple effects on the wing surface. None of the prior art devices or arrangements is capable of also cleaning and/or rinsing the aircraft--in addition to de-icing and anti-icing the aircraft--and in one "pass" of the aircraft through a bidirectional single integrated apparatus. None of the prior art devices and/or arrangements provides an efficient means by which the underside of the aircraft (including the underwing surfaces thereof) may be simultaneously de-iced and anti-iced (and/or cleaned and rinsed) and, again, in one "pass" of the aircraft through a bidirectional single integrated apparatus.
Accordingly, it will be appreciated that there exists a long-standing need for an apparatus (and a method) for de-icing and anti-icing (and/or cleaning and rinsing) a wide variety of aircraft, and wherein the apparatus has a relatively-low capital investment, is efficient, conserves expensive glycols, is rugged and reliable, easy to service and to maintain, saves manpower and time, protects the environment, allows aircraft of all types to be quickly de-iced and anti-iced, and (most importantly) assures the highest margin of aircraft safety under adverse weather conditions while maintaining flight schedules.